Ultra-wideband (UWB) is a term for a classification of signals that occupy a substantial bandwidth relative to their centre frequencies (for example, according to the Federal Communications Commission (FCC) Rules, UWB signal is a signal whose fractional bandwidth (the ratio between its bandwidth to its center frequency) is equal to or greater than 0.2, or its bandwidth is equal to or greater than 500 MHz.). This bandwidth corresponds in the time domain to very short pulses and allows UWB based radar systems to obtain more accurate target information, and makes it possible to design radars with improved spatial resolution compared to conventional radar. Therefore UWB radar devices are common for through-the-obstacle radar-based imaging systems enabling information gathering through obstacles such as walls, doors, ground, smoke, vegetation and other visually obstructing substances and, as such, may be invaluable for Intelligence, Surveillance, and Reconnaissance (ISR) applications, including military, counter-terrorism forces and law enforcement applications. These applications require effective volume visualization based on obtained signals.
The terms “volume visualization” and “image reconstruction” used in this patent specification include any kind of image-processing, volume rendering or other image-reconstructing computing used to facilitate displaying three-dimensional (3D) data on a two-dimensional (2D) image surface.
The radar-based image systems can use different types of radars. While in mono-static radar systems transmitter and receiver can share a common antenna, bi-static radar comprises a transmitter and receiver which are spatially separated. The relative distance is comparable to the expected target distance, thereby enabling receiving a signal also when the geometry of the reflecting object reflects very little or no energy in the direction of the mono-static radar. Multi-static radar systems include multiple spatially diverse mono-static radar or bi-static radar components with a shared area of coverage. The spatial diversity utilized by multi-static systems allows for simultaneously receiving different aspects of the scanned target.
The problems of volume visualization in multi-static radar image systems have been recognized in the contemporary art and various systems have been developed to provide a solution, for example:
US Patent Application No. 2003/074812 (Stump) discloses a portable structure supporting a subsurface imaging system and moveable over a given imaging site. At least a first antenna of a plurality of antennae is oriented in a manner differing from an orientation of a second antenna of the plurality of antennae, such as the first antenna being orientated substantially orthogonal to the second antenna. The antennae may operate in a bi-static mode. Transmitter and receiver circuitry, coupled to the antennae, respectively generates electromagnetic probe signals and receives electromagnetic return signals resulting from the probe signals. A processor processes the received electromagnetic return signals. A display may be provided as part of the subsurface imaging system and/or as part of a processing system separate from the subsurface imaging system which processes the received electromagnetic return signals. The processor can generate two-dimensional and/or three-dimensional detection data using the received electromagnetic return signals.
US Application No. 2003/043067 (Johansson et al.) discloses methods and systems capable of identifying a buried object using array-based ground penetrating radar having a control device, a plurality of transmit antennas, and a plurality of receive antennas. Such methods and systems receive a transmit timing input signal and a receive timing input signal. Such methods and systems comprise a first delay circuit for receiving the transmit timing input signal and generating a number of intermediate transmit timing signals delayed with respect to each other by a delay time, and transmit output switch circuit to select either the transmit timing input signal or a corresponding one of the intermediate transmit timing signals as a corresponding output transmit timing signal. Such methods and systems also comprise a second delay circuit for receiving the receive timing input signal and generating a number of intermediate receive timing signals delayed with respect to each other by the delay time, a shift-delay circuit coupled to the second delay circuit and the receive timing input signal to add the delay time to the intermediate receive timing signals, and a receive output switch circuit to select either the receive timing input signal or a corresponding one of the intermediate receive timing signals as a corresponding output receive timing signal. Such methods and systems also comprise an antenna array comprising a plurality of transmit antennas, a plurality of receive antennas, and means for selectively enabling the transmit and receive antennas to allow each of the receive antennas to receive energy from any one of the transmit antennas.
U.S. Pat. No. 6,031,485 (Cellai et al.) discloses a hi-static spread spectrum digital radar including a transmitting antenna and a receiving antenna array separate from and positioned at a distance from the transmitting antenna. The transmitting antenna transmits a pseudo random digital signal as produced by a pseudo random digital signal generator. The receiving antenna array receives signals simultaneously and in parallel as reflected from a target by the transmitting antenna. Analog-to-digital converters are connected respectively to each element of the receiving antenna array so as to convert the received analog signal into a digital signal. Digital beams are produced and directed to a set of digital receivers. A processor is provided to produce a digital output from the set of digital receivers.
US Patent Application No. 2009/271146 (Ammar) discloses concealed object detection using electromagnetic and acoustic multi-static imaging systems and methods. A method of simultaneously screening plural subjects for concealed objects includes transmitting a signal into a screening area where there is at least one subject to be screened having an associated object, receiving a reflected signal from the object when the object is located within the screening area, processing the reflected signal using multi-static Fourier space sampling and tomographic reconstruction to generate a three-dimensional image of the object and displaying the three-dimensional image. The transmitting and receiving are performed using a multi-directional array including at least three sensors. An object detection system includes a screening area, a multi-directional array including at least three sensors, a processor configured to execute multi-static Fourier space sampling and tomographic reconstruction and a display.
US Patent Application No. 2010/207804 (Hayward et al.) discloses a method and system for locating objects in a region having a high degree of multipath susceptibility comprising a plurality of transducers, each being a transmit or receive antenna, and being arranged about the region in known locations, to form a bi-static or multi-static radar, with some embodiments being MIMO systems. Signals transmitted by the transmit antenna(s) are received at each receive antenna and processed to form a set of channel impulse responses, or power delay profiles, representative of the region at a given time. A second set is formed at a different time, and the difference between the two sets is calculated, the difference containing information on movement within the region. The difference may be processed to localize the moving object(s) by graphical means, e.g. by generating an x-y image representative of the region, and accumulating at each pixel appropriate values of the profile difference selected according to the propagation delay between a transmit-receive antenna pair via a region point represented by the pixel.
US Patent Application No. 2011/025546 (Cook et al.) discloses a method and apparatus for sensing a target through a wall or obstruction by a Moving Target Indicator (MTI) radar sensor. In an exemplary embodiment, a series of radar pulses are transmitted at frequencies less than about 5 GHz. Radar return signals are received at a plurality of receive antenna array sub-apertures. The radar return signals are processed by a digital beam-former to form multiple beams. Target detection processing detects moving and stationary targets through a plurality of parallel target detection signal processing paths.